Calculating Percentage by Mass

• What should you be able to calculate by the end of the video?

  The percentage by mass of an element in a compound
• What does the absence of a small number to the right of an element's symbol indicate in a compound?

  It indicates that there is one atom of that element in the compound
• What are the elements present in magnesium oxide?

  Magnesium and oxygen
• How do you calculate the relative formula mass of a compound?

  By adding together the relative atomic masses of the atoms in the formula
• What is the symbol for relative formula mass?

  m r
• What is the relative atomic mass of magnesium?

  24
• What is the relative atomic mass of oxygen?

  16
• What is the relative formula mass of magnesium oxide?

  40
• What does the percentage by mass tell us?

  What percentage of the relative formula mass is due to one of the elements in a compound
• How do you calculate the percentage by mass of an element?

  By dividing the total relative atomic mass of the element by the relative formula mass of the compound and multiplying by 100
• What is the percentage by mass of magnesium in magnesium oxide?

  60%
• What is the formula for calcium chloride?

  CaCl₂
• What is the relative formula mass of calcium chloride?


  111
• What is the relative atomic mass of calcium?

  40
• What is the percentage by mass of calcium in calcium chloride?

  36%
• What is the formula for methane?

  CH₄
• What is the relative formula mass of methane?

  16
• What is the relative atomic mass of hydrogen?

  1
• What is the percentage by mass of hydrogen in methane?

  25%
• What is the formula for sodium sulfate?

  Na₂SO₄
• What is the relative formula mass of sodium sulfate?

  142
• What is the relative atomic mass of sodium?

  23
• What is the percentage by mass of sodium in sodium sulfate to one decimal place?

  32.4%
• What is the process to calculate the percentage by mass of an element in a compound?

  1. Identify the total relative atomic mass of the element.
  2. Identify the relative formula mass of the compound.
  3. Use the formula:

  \[

  \text{Percentage by mass} = \left( \frac{\text{Total relative atomic mass of the element}}{\text{Relative formula mass of the compound}} \right) \times 100

  \]
• What are the steps to calculate the percentage by mass for different compounds mentioned?

  • For magnesium in magnesium oxide:
  • Total atomic mass = 24
  • Relative formula mass = 40
  • Percentage = 60%

  
  

  • For calcium in calcium chloride:
  • Total atomic mass = 40
  • Relative formula mass = 111
  • Percentage = 36%

  
  

  • For hydrogen in methane:
  • Total atomic mass = 4
  • Relative formula mass = 16
  • Percentage = 25%

  
  

  • For sodium in sodium sulfate:
  • Total atomic mass = 46
  • Relative formula mass = 142
  • Percentage = 32.4%